Although there have been numerous studies on the early phase of myocardial infarction the determinants of the healing phase of infarction are largely unknown. Previous studies which have assessed the healing phase of infarction mainly have been qualitative descriptions of autopsy material; a systematic examination of what factors affect healing is lacking. Poor scar formation could lead to aneurysmal bulging with infarct expansion, deterioration of cardiac function and cardiac rupture. The purpose of this study is to characterize the healing phase of canine myocardial infarction by assessing a) in vivo (with ultrasonic crystals) and in vitro (by pathologic assessment) infarct morphology and scar thickness and evidence of infarct expansion, b) behavior of infarcted myocardium and global left ventricular function over time; c) myocardial scar compliance (as assessed by ultrasonic crystals and passive pressure-length measurements) and d) tensile strength of the developing scar. The effect of interventions which could potentially affect the healing phase of infarction will be assessed. These include drugs which experimentally reduce infarct size (nonsteroidal and steroidal antiinflammatory agents), antiplatelet agents, inotropic and negative inotropic drugs. In addition, the effect of coronary reperfusion during acute myocardial infarction (as might occur in patients receiving intracoronary streptokinase) on the healing phase will be assessed. To determine whether exercise following myocardial infarction alters the healing phase of infarction and results in infarct expansion, a rat model of coronary occlusion will be used.